Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 739

soil concentration and (usually) soil Ca-concentration. Including pH in the regres-
sions occasionally improved the predictive power of the equations. Nahmani et al.
( 2009 ) applied a selection of these equations and those derived by Neuhauser et al.
( 1995 ) and Peijnenburg et al. ( 1999 ) to their own data set that they had used to deter-
mine rates of metal uptake. The equations of Sample et al. ( 1999 ) were found to best
predict tissue metal concentrations.
16.4.2.2 Organic Contaminants
As with metals, for organic contaminants a distinction can be made between statisti-
cal models linking accumulated contaminants to extractable concentrations or pore
water contents, and more mechanistically based modelling approaches like the one
reported by Jager ( 1998 ) for estimating bioconcentration in earthworms.
Three conceptual frameworks provide the basic concepts for modelling bioavail-
ability of contaminants. The first conceptual framework is the concept of chemical
equilibrium in which chemical activities (or fugacities) are the driving factor for
transport and distribution processes, including passive uptake of contaminants by
biota (Diamond et al. 1992 ). The fugacity concept dictates chemical fugacities to
be similar across biological membranes and explains observed variability in uptake
patterns for organisms for which active uptake (like feeding and ingestion of solid
soil particles) is of importance. Similarly, the concept explains why deviations from
pore water uptake are often observed for highly hydrophobic contaminants (i.e.,
with log-transformed values of the octanol-water partition coefficient > approxi-
mately 5). Within the fugacity concept, Reichenberg and Mayer ( 2006 ) identified
two complementary aspects of bioavailability of organic contaminants, these being
the accessible quantity and the chemical activity that is to be deduced from this
quantity as related to the physico-chemical conditions of the soil.
The second conceptual framework is the concept of equilibrium partitioning the-
ory (EPT) in which chemical activities in the pore water are assumed to drive uptake
and effects (Van Gestel 1997 ). The equilibrium partitioning concept is schematically
given in Fig. 16.2 . EPT assumes that the major distribution processes in the soil
compartment, i.e., between soil - pore water - biota, can be described or predicted
from simple physico-chemical properties such as the lipophilicity of the organic
contaminant and the relative amount of binding sites in the soil or fat content of
the biota. The EPT can be used to predict body residues and/or toxicity in soil
dwelling species on the basis of data generated with aquatic organisms assuming
that either pore water is the only route of exposure or that additional uptake path-
ways are proportional to pore water uptake, and that aquatic species generally have
the same overall sensitivity distribution as terrestrial species. Within the basic EPT
concept it is explicitly realized that the morphology, physiology and behaviour of
biota dominate actual uptake and effects.
The third and most general concept is the general concept of bioavailability
advocated by the ISO-working group on bioavailability (Harmsen 2007 ;ISO/DIS
2006 ). This group established a general bioavailability scheme for both organic and
inorganic contaminants.